System and method for electrically-coupled thermal cycle

ABSTRACT

In one embodiment according to the invention, there is provided a method for generating electrical energy using a thermal cycle of a working gas. The method comprises using the motion of a piston in a cylinder, containing the working gas performing the thermal cycle, to electromagnetically induce current in an electrical circuit coupled to the cylinder; using the electrical circuit to store the electrical energy, produced by the current induced in the electrical circuit, in an electrical storage device; and using the electrical energy stored in the electrical storage device to electromagnetically provide a motive force to the piston. Cyclically using the electrical circuit to store the electrical energy and using the stored energy to provide a motive force to the piston effect a net positive average power transfer into the electrical storage device over the course of the thermal cycle.

BACKGROUND OF THE INVENTION

A thermal cycle of a heat engine that employs a quantity of gas as anoperating medium can be described by reference to a pressure-volume(P-V) diagram. FIGS. 1 and 2 show P-V diagrams for two well-knownthermal cycles, the Carnot cycle (FIG. 1), and the ideal Sterling cycle(FIG. 2).

The net energy delivered from one thermal cycle is the area of the loopswept out by the operating path in the P-V plane. In the course of eachcycle, energy is delivered by the engine for part of the cycle, and isabsorbed by the engine for the remainder of the cycle. For some parts ofsome cycles, energy is neither stored nor delivered. For instance, inthe ideal Sterling cycle, mechanical energy is neither absorbed nordelivered during those parts of the cycle where the trajectory isparallel to the P-axis.

By necessity, part of the system used for extracting a net positiveaverage power output must include a device for storing and returningenergy out of and into the heat engine, on a cyclic basis. Inconventional heat engines, this cyclic energy storage is accomplished bymechanical means, for example via the rotational inertia of a crankshaftwith flywheel attached.

SUMMARY OF THE INVENTION

It is desirable to be able to convert heat into electricity by means ofa method in which the equipment is reliable, efficient, quiet, free ofvibration, and capable of operating from a variety of fuels.

It is also desirable to be able to use electricity to effect heattransfer by means of equipment with such attributes.

To achieve these and other objectives, an embodiment of the inventionprovides a method for generating electrical energy using a thermal cycleof a working gas. The method comprises using the motion of a piston in acylinder, containing the working gas performing the thermal cycle, toelectromagnetically induce current in an electrical circuit coupled tothe cylinder. The electrical circuit is used to store the electricalenergy, produced by the current induced in the electrical circuit, in anelectrical storage device; and the electrical energy stored in theelectrical storage device is used to electromagnetically provide amotive force to the piston. Cyclically using the electrical circuit tostore the electrical energy and using the stored energy to provide amotive force to the piston effect a net positive average power transferinto the electrical storage device over the course of the thermal cycle.

The electrical circuit may comprise an electronic power converter, andthe method may further comprise using the electronic power converter toperform closed-loop electronic control of the motion of the piston. Theelectronic power converter may perform the closed-loop control based onelectrical signals related to the state of the working gas. At least oneof a temperature sensor, a pressure sensor, and a position sensor may beused to deliver the electrical signals related to the state of theworking gas to the electronic power converter.

The thermal cycle may approximate a Sterling cycle, a Carnot cycle, anOtto cycle, or another thermal cycle. The thermal cycle may receive heatfrom external combustion, or the working gas may be cycled through aninternal combustion cycle.

Compression and expansion of the working gas between a first piston anda second piston may be used to perform the thermal cycle. The electricalcircuit may comprise a set of windings coupled to the cylinder, and themethod may comprise using the motions of a first permanent magnetattached to the first piston and a second permanent magnet attached tothe second piston to electromagnetically induce current in the set ofwindings. Further, the motions of the first piston and the second pistonmay be used to move the working gas along the cylinder to effectsuccessive heat transfer with a heating zone and a cooling zone of thecylinder.

At least part of the shaft of the first piston may move concentricallywithin a shaft of the second piston. The electronic power converter maybe used to control timing of the thermal cycle by controlling themotions of the first piston and the second piston; including bycontrolling the motions of the first piston and the second piston suchthat the working gas moves between a heating zone, a cooling zone, and aneutral zone of the cylinder. A thermal shade may be attached to thefirst piston or the second piston to insulate non-working gas within thecylinder; and a paddle may be attached to the first piston or the secondpiston to create turbulence in the working gas. An external flow returnmay be used to flow non-working gas between a first end zone and asecond end zone of the cylinder. The first piston and the second pistonmay be mounted around a common centering shaft.

Two cylinders operating according to the invention may be operated inaxial opposition to each other. Similarly, four cylinders may beoperated in a bundle with parallel axes of the cylinders, two of thecylinders being operated antiparallel to the other two cylinders of thebundle.

In another embodiment according to the invention, there is provided amethod for powering a heat pump using electrical energy, the heat pumpperforming a thermal cycle. The method comprises using electrical energystored in an electrical storage device to electromagnetically provide amotive force to a piston in a cylinder containing the working gasperforming the thermal cycle. The motion of the piston is used toelectromagnetically induce current in an electrical circuit coupled tothe cylinder; and the electrical circuit is used to store the electricalenergy, produced by the current induced in the electrical circuit, inthe electrical storage device. Cyclically using the stored energy toprovide the motive force to the piston and using the electrical circuitto store the electrical energy effect a net positive average powertransfer out of the electrical storage device over the course of thethermal cycle. Similar methods as those used with the method forgenerating electrical energy, above, may be used with the method forpowering a heat pump.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 shows a pressure-volume diagram for a Carnot cycle, known in theart;

FIG. 2 shows a pressure-volume diagram for an ideal Sterling cycle,known in the art;

FIG. 3A shows an arrangement of coils, magnets, and pistons for anexternal combustion cylinder according to an embodiment of theinvention;

FIG. 3B shows a separate view of a piston for the embodiment of FIG. 3A;

FIG. 4 is a schematic diagram of electrical components that are coupledto the external combustion cylinder arrangement of FIGS. 3A-3B;

FIG. 5A illustrates an alternative embodiment that may be used in placeof the mechanical arrangement of FIG. 3A, in accordance with anembodiment of the invention;

FIGS. 5B and 5C show separate views of pistons for the embodiment ofFIG. 5A;

FIG. 6 is a timing diagram for the heat engines of FIGS. 3A and 5A whenoperated as electricity generators per the Sterling cycle depicted inFIG. 2, in accordance with an embodiment of the invention;

FIG. 7 is a P-V diagram for a Sterling cycle heat pump operated inaccordance with an embodiment of the invention;

FIG. 8 is a timing diagram for the Sterling cycle heat pump of FIG. 7;

FIG. 9 illustrates an alternative embodiment that may be used in placeof the mechanical arrangements of FIGS. 3A-3B and 5A-5C, in accordancewith an embodiment of the invention;

FIG. 10 shows an axially opposed heat engine according to an embodimentof the invention;

FIGS. 11A and 11B illustrate an arrangement of four of the cylinderassemblies of the type shown in FIG. 5A placed side-by-side withparallel central axes, according to an embodiment of the invention;

FIG. 12 is a timing diagram for the heat engines of FIGS. 3A and 5A whenoperated as electricity generators per the Carnot cycle depicted in FIG.1, in accordance with an embodiment of the invention;

FIG. 13 is a cross-sectional view of a piston arrangement for aninternal combustion generator, in accordance with an embodiment of theinvention;

FIG. 14 is a timing diagram for an internal combustion generator, inaccordance with an embodiment of the invention; and

FIG. 15 is a P-V diagram of an Otto cycle by which an internalcombustion generator may be operated, in accordance with an embodimentof the invention.

DETAILED DESCRIPTION OF THE INVENTION

Rotational inertia has been the method of choice for cyclic energystorage in heat engines since their development in the eighteenthcentury. Thus, the devices used for cyclically storing and returningenergy out of and into the heat engine are typically mechanical. Forexample, an engine may use the rotational inertia of a crankshaft withflywheel attached for cyclical energy storage. In this way, conventionalheat engines can be said to use mechanically-coupled thermal cycles.

However, in such a mechanically-coupled thermal cycle the motion of thepistons is constrained by the motion of the crankshaft. The pistonstherefore cannot move in a manner that allows the state of the workinggas to closely follow the desired P-V cycle. The relative amounts oftime devoted to each segment of the cycle are fixed by the mechanicalconstraints on the motion of the flywheel. Moreover,mechanically-coupled heat engines are constrained in their reliabilityand efficiency, the amount of noise and vibration they generate, andtheir ability to operate from a variety of fuels.

In order to improve on these characteristics, an embodiment according tothe invention uses an electricity storage device to accommodate thecyclic flow of energy from a thermal cycle. The thermal cycle cantherefore be described as electrically-coupled.

An embodiment uses direct electric drive of pistons by means ofelectromagnetic shear.

Electricity storage devices suitable for this application include, forexample, capacitors, batteries, and (if available) superconductingcoils. Direct electric drive using electromagnetic shear may beaccomplished with the use of permanent magnets attached to each pistonassembly, and with the use of controlled electric currents in coils orwindings to provide force to, or electromagnetic induction from, thepermanent magnets.

Embodiments of an electrically-coupled thermal cycle may be used for thegeneration of electricity from a thermal cycle, such as to charge abattery using the external or internal combustion of a gas; or forelectrical powering of a thermal cycle, such as using a battery or othersource of direct current to power a heat pump.

In accordance with the invention, power electronic circuits can be builtwhich permit the motion of the pistons to be controlled so as to followas closely as possible any desired path in the P-V plane. The necessaryenergy cycling required to extract average power from a heat engine canbe effected via electrical energy storage. The use of electric couplingin this manner allows for variation of the amounts of time spent in eachsegment of a P-V cycle, thereby allowing for high thermal cycleefficiencies.

Therefore, by comparison with prior systems in which energy wascyclically stored mechanically, an embodiment according to the inventionuses electrical storage of cyclical energy flow. In addition, use ofelectrical circuitry allows closed-loop electrical control of pistonmotion.

In the prior art, refrigeration devices are known that are driven byelectronic linear drive motors, such as in U.S. Pat. No. 4,761,960 ofHigham et al.; U.S. Pat. No. 4,697,113 of Young; and U.S. Pat. No.5,040,372 of Higham. Further, such linear drive motors may bebattery-powered, with the delivery of current from the battery beingelectrically controlled, as in U.S. Pat. No. 5,752,385 of Nelson andU.S. Pat. No. 4,434,617 of Walsh. Also, free-piston hydraulic enginesare known, such as in U.S. Pat. No. 4,215,548 of Beremand.

However, an embodiment according to the present invention isfundamentally different from such previously known systems because itemploys electrical storage of cyclical energy flows to and from thethermal cycle. Thus, within a thermal cycle, an embodiment according tothe invention cycles energy into and out of an electrical storage devicethat is electrically coupled to a cylinder containing the piston. Bycontrast, such previously known systems did not use electrical storageof cyclical energy flow. Some such prior systems may instead use a formof mechanical resonance for cyclical energy flow. For example, in U.S.Pat. No. 4,434,617, a mechanical resonance is used between the mass ofthe piston and the compressed end-zone gas, which acts as a spring, forcyclical energy flow. Although a synchronized electrical drive is usedto assist and maintain the mechanical resonance, the system does not usean electrical storage device to absorb the cyclical energy flow from thethermal cycle. Such systems therefore do not allow the potentialimprovements in thermal efficiency provided by using electrical storageof cyclical energy flows from a thermal cycle, and electronic control ofthe cyclical energy flows, according to an embodiment of the invention.

A description of preferred embodiments of the invention follows.

FIGS. 3A and 3B show an arrangement of coils, magnets, and pistons foran external combustion generator according to an embodiment of theinvention. In the cross-sectional view of FIG. 3A, a closed gascontainment cylinder 301 contains a body of gas, a portion of whichbecomes the working gas 302. The working gas 302 is the subset of thetotal gas within the cylinder 301 that lies between two pistons 303 and304, which slide within the cylinder 301. The pistons 303 and 304maintain a tolerably good gas seal with the inner wall of the cylinder301 without creating undue friction. Conventional piston rings, forexample, may be employed for this purpose. The cylinder 301 willtypically be of circular cross section, but may have other crosssectional shapes. The working gas 302 may be any gas suitable for thepurpose, such as air, nitrogen, helium, or hydrogen.

As shown in FIG. 3B, each of the two pistons 303 and 304 is in the formof a plate. Attached centrally and perpendicular to each plate 303 and304 is a shaft 307 and 308 attached at its other end to a permanentmagnet plate 305 and 306. The permanent magnet plates 305 and 306contain permanent magnets within them, suitably arranged, together withmagnetic path material such as iron or a suitable grade of steel. Thearrangement of the permanent magnets and magnetic path material is suchas to produce magnetic flux emanating from the outer edges of thepermanent magnet plates 305 and 306, which cuts through the drivewindings 309 and 310 surrounding the cylinder 301 (FIG. 3A). Thecylinder 301 is made of nonmagnetic materials. A plurality of suchmaterials may be employed to construct cylinder 301. For example, amaterial such as aluminum may be used for regions such as 313 and 314,where no heat flow is required; and a material such as ceramic orfiberglass may be used for regions such as 317, where heat flow is notrequired. Surrounding the drive windings 309 and 310 are magnetic fieldreturn paths 311 and 312 made of magnetic path material.

Also surrounding the cylinder 301 are two heat transfer zones 313 and314 made of thermally conductive material such as copper or an aluminumalloy. A heating zone 313 accepts heat from an external heat source, forexample a flame or solar collector, and transfers that heat into theworking gas 302 at an appropriate time, as described below. Likewise, acooling zone 314 extracts heat from the working gas 302 at anappropriate time, also described below. The heat transfer zones 313 and314 are separated from each other by a thermally insulated neutral zone317. The three zones 313, 314, and 317 are shown in the accompanyingfigures to be of comparable lengths, which is not necessarily required,but may be advantageous with regard to optimization of overall poweroutput.

FIG. 4 is a schematic diagram of electrical components that are coupledto the external combustion cylinder arrangement of FIGS. 3A-3B, in orderto accommodate the cyclic flow of energy from the thermal cycle inaccordance with an embodiment of the invention. Drive windings 409, 410,443, which are the drive windings depicted as 309, 310 of FIG. 3A,connect to an electronic power converter 435. FIG. 4 shows threeisolated windings 409, 410, 443 for illustrative convenience, but anynumber of separate windings may be employed, as necessary. Alsoconnected to the electronic power converter 435 are signals fromposition sensors 436, a temperature sensor 440, and a pressure sensor441. It will be appreciated that any appropriate number of suchposition, temperature, and pressure sensors may be employed. Theposition sensors 436 give the electronic power converter 435 theinformation it needs to know the exact location of each piston at anyinstant in time. The temperature sensor 440 and pressure sensor 441inform the electronic power converter 435 of the state of the workinggas 302 at any instant.

Electronic power converter 435 is connected to a DC Bus 442, to which isconnected a capacitor 437 and/or a battery 438, and an electric load439. The electric load 439 may be disconnected from the DC Bus 442 whennot required, while the electronic power converter 435 continues tocharge the battery 438. Suitable batteries for battery 438 includelithium or other modem types of batteries configured for energy cyclingapplications, with better performance gained by lithium or other typesof batteries capable of cycling energy at a rate of a few cycles persecond or faster.

During operation of the system, the electronic power converter 435 ofFIG. 4 controls the flow of electric current into and out of thewindings 309 and 310 of FIG. 3A such that pistons 303 and 304 move upand down within the cylinder 301 to cause the working gas 302 to followa desired P-V cycle. The capacitor 437 and battery 438 act as the energyreservoir for the system, and absorb the cyclic energy variations whichare integral to the cycles of heat engines. The electronic powerconverter 435 stores little or no energy, and transfers power betweenthe DC Bus 442 and the windings 309 and 310 in a highly efficientmanner.

In this way, the embodiment of FIGS. 3A-4 provides anelectrically-coupled external combustion generator. Energy released fromexternal combustion is transferred into the cylinder 301 through heatingzone 313, a pressure-volume cycle is produced in working gas 302, andcyclic energy storage is performed by the electrical circuitry of FIG.4. In one application, for example, the external combustion of a gas maytherefore be used to store electrical charge in battery 438 withoutusing any moving parts other than the pistons 303 and 304.

FIGS. 5A-5C illustrate an alternative embodiment that may be used inplace of the mechanical arrangement of FIGS. 3A-3B, wherein the drivewindings 509 and 510 are placed adjacent to each other and away from theheating zone 513. By placing the permanent magnet plates 505 and 506away from the heating zone 513, this arrangement simplifies the designtask of keeping the permanent magnets cool. Neodymium-iron permanentmagnet material loses its magnetism when subjected to high temperatures,and is limited to working temperatures typically no higher than 150 to200 C. As shown in FIG. 5A, the shaft 507 of a longer piston assembly(shown separately in FIG. 5B) lies concentrically within the shaft 508of a shorter piston assembly (shown separately in FIG. 5C). Themechanical fit between these two shafts 507 and 508 is such as to give atolerably good gas seal between them without creating undue friction.The inner shaft 507, which connects piston 503 to its permanent magnetplate 505, is constructed to give minimal heat conduction from the hotupper end 503 to the permanent magnet plate 505 and to the shaft 508surrounding it. This may be effected by using a thermally insulatingmaterial such as ceramic for shaft 507, possibly with a metallic corefor strength. The drive windings 509 for the longer piston assembly 503are located further away from the cooling zone 514 than the drivewindings 510 for the shorter piston assembly 504. In FIG. 5A (unlikewith plates 305 and 306 in FIG. 3A), permanent magnet plate 506 islocated above permanent magnet plate 505, because piston assembly 503 islonger than piston assembly 504. The operation of the heat enginedepicted in FIG. 5A is just as described above for the heat enginedepicted in FIG. 3A, with similar electrical coupling to circuitry suchas that of FIG. 4.

FIG. 6 is a timing diagram for the heat engines of FIGS. 3A and 5A whenoperated as electricity generators per the Sterling cycle depicted inFIG. 2, in accordance with an embodiment of the invention. Curve 644 isthe piston position profile for piston 303, 503, and curve 645 is thepiston position profile for piston 304, 504, for a repeating cycleA-B-C-D-A. The piston positions are indicated by position levels 0through 3 on the y-axis of FIG. 6, which correspond to cylinderpositions indicated in FIGS. 3A and 5A. The cooling zone 314, 514extends from position level 0 to level 1; the neutral zone 317, 517extends from position level 1 to level 2; and the heating zone 313, 513extends from position level 2 to level 3. Although FIG. 6 shows theamount of time spent in each of the four segments of the thermal cycleas approximately equal, it is to be understood that the duration of eachsegment can be varied independently of the others, thereby allowing forpower output variation and efficiency maximization. In varying theduration of the segments, there is an inherent conflict between theobjectives of maximizing power output and maximizing efficiency; eitherobjective can be satisfied, but not both simultaneously.

Between times A and B of FIG. 6 the working gas 302, 502 is compressedat constant temperature T1. In the A-B path, piston 304, 504 is held atposition Level 0 (shown on the y-axis of FIG. 6, and in FIGS. 3A and 5A)as shown by curve 645, while piston 303, 503 is moved from positionLevel 2 to Level 1 as shown by curve 644, thereby compressing theworking gas 302, 502. The motion 644 of piston 303, 503 for this segmentis depicted as having a straight-line shape in FIG. 6, although inpractice the motion will typically be nonlinear.

Between times B and C of FIG. 6, the working gas 302, 502 is held atconstant volume and heated to temperature T2. In the B-C path, bothpistons initially move quickly together such that piston 303, 503 ismoved from position Level 1 to Level 3, while piston 304, 504 is movedfrom position Level 0 to Level 2, as indicated by curves 644 and 645.For the duration of the B-C time segment, piston 304, 504 is held atposition Level 2 (curve 645), and piston 303, 503 is held at positionLevel 3 (curve 644).

Between times C and D of FIG. 6, the working gas 302, 502 expands atconstant temperature T2. In the C-D path, piston 303, 503 is held atposition Level 3 (curve 644), while piston 304, 504 is moved fromposition Level 2 to Level 1 (curve 645). The motion of piston 304, 504for this segment is depicted as having a straight-line shape in curve645 of FIG. 6, although in practice the motion will typically benonlinear.

Between times D and A of FIG. 6, the working gas 302, 502 is again heldat constant volume and is cooled to temperature T1. In the D-A path,both pistons initially move quickly together such that piston 303, 503is moved from position Level 3 to Level 2 (curve 644), while piston 304is moved from position Level 1 to Level 0 (curve 645). For the durationof the D-A time segment, piston 304, 504 is held at position Level 0(curve 645), and piston 303, 503 is held at position Level 2 (curve644).

Examination of the timing diagram of FIG. 6 shows that there areportions of the cycle wherein the pistons are stationary. These regionsmay afford an opportunity for efficiency improvement, whereby amechanical means is used to hold each piston in its appointed placeduring a stationary portion of the cycle rather than relying on the flowof electric current in the drive windings, with its attendant ohmiclosses. For instance, during the compression region A-B in FIG. 6,piston 304, 504 could be prevented from moving even lower than positionLevel 0 by a mechanical impediment. The state of pressure in the endzones 315/515, 316/516 will be a factor in the implementation of thistechnique, and the design of the end zones may need to be modifiedaccordingly.

Such mechanical hard stops could, in principle, take the form of amechanical barrier, or they may be effected by means of permanentmagnets (and/or magnetic poles) attached rigidly either to the cylinder301, 501 and/or to the piston assemblies. If a mechanical barrier isused, the power electronics can control the motion of the piston as itapproaches the barrier so as to effect a “soft landing”. A soft, springymaterial attached to the barrier or to the piston may assist withensuring a soft landing. Permanent magnets would have the advantage ofmaintaining a physical impediment to further motion, without thepractical concerns of physical contact associated with mechanicalbarriers. The permanent magnets can be used either in the attractionmode or in the repulsion mode. If they are used in the attraction mode,the control electronics will need to provide an excess impulse ofcurrent in order to break the piston free from its magnetic confinementat the end of the stationary period.

Although a Sterling cycle has been described above, the generalarrangement illustrated by FIGS. 3A-5C can be used for other types ofthermal cycle, including one which approximates the Camot cycle ofFIG. 1. Such a Camot Engine may operate, for example, via the timingdiagram of FIG. 12, which applies to a physical arrangement in which thelength of the neutral zone 317, 517 is three times that of the heating313, 513 and cooling 314, 514 zones. In FIG. 12, curve 1244 is thepiston position profile for piston 303, 503, and curve 1245 is thepiston position profile for piston 304, 504, for a repeating cycleA-B-C-D-A. Because of the extended length of the neutral zone 317, 517,the position levels are shown ranging from level 0 to level 5, with thecooling zone 314, 514 extending from level 0 to level 1, the neutralzone 317, 517 extending from level 1 to level 4, and the heating zone313, 513 extending from level 4 to level 5. Time interval A-Bcorresponds to isothermal compression, interval B-C correspondsadiabatic compression, interval C-D corresponds to isothermal expansion,and interval D-A corresponds to adiabatic expansion.

While the embodiments of FIGS. 3A-5C have been described as generators,by which heat is converted to electricity, it is also possible to use anelectrically-coupled thermal cycle in accordance with an embodiment ofthe invention to create an electrically-powered heat pump. In this case,the embodiments of FIGS. 3A-5C are essentially operated in reverse:energy stored in electrical circuitry such as that of FIG. 4 is cycledin and out of a cylinder 301, 501 via windings 309, 509 and 310, 510, sothat the pistons 303-304 and 503-504 perform a heat pump cycle. Such aheat pump may be used to generate heat or to receive heat, which can betransferred to or from an external object through heating and coolingzones 313-314 and 513-514.

FIG. 7 shows a P-V diagram for such a Sterling cycle heat pump (i.e., arefrigerator) operated in accordance with an embodiment of theinvention. It can be seen that the path followed is that of FIG. 2,taken in reverse. FIG. 8 gives the corresponding timing diagram, whichcan be understood by reference to the similar preceding explanation forFIG. 6. Curve 844 is the piston position profile for piston 303, 503,and curve 845 is the piston position profile for piston 304, 504, for arepeating cycle A-B-C-D-A. The piston positions are indicated byposition levels 0 through 3 on the y-axis of FIG. 8, which correspond tocylinder positions indicated in FIGS. 3A and 5A. The cooling zone 314,514 extends from position level 0 to level 1; the neutral zone 317, 517extends from position level 1 to level 2; and the heating zone 313, 513extends from position level 2 to level 3.

FIG. 9 shows an alternative embodiment that may be used in place of themechanical arrangements of FIGS. 3A-3B and 5A-5C. A centering shaft 921is located centrally and within the shaft 907 of piston assembly 903,which in turn is located within the shaft 908 of piston assembly 904.Again, the mechanical fit between the shafts 921, 907, and 908 is suchas to give a tolerably good gas seal between them without creating unduefriction. Centering shaft 921 holds both piston assemblies 903, 904centered within the cylinder 901, so that they do not cling to one sideof the cylinder via magnetic attraction, thereby causing excess frictionand compromised gas sealing. The centering shaft 921 therefore assiststo improve system efficiency.

In an alternative embodiment according to the invention, portions ofcentering shaft 921 may be made of magnetic path material encircled byfield coils, to create an electromagnet, thereby providing a means forthe elimination of permanent magnets in the plates 305 and 306 of FIG.3A. For example, using field coils wound around each end of such amagnetic centering shaft 921, two electromagnets may be created, toreplace the functional role of permanent magnets in plates 305 and 306.Plates 305 and 306 are then made of magnetic path material.

Returning to the embodiment of FIG. 9, thermal shades 922 can be fittedto, or made part of, the pistons 903 and 904. The function of thesethermal shades 922 is to impede the flow of heat through the heating 913and cooling 914 zones during appropriate portions of the heat cycle. Thethermal shades 922 are made of thermally insulating material, and arelocated close to, but not in contact with, the inside walls of thecylinder 901. The thermal shades 922 extend around the entire innerperimeter of the inside walls of the cylinder 901. They impede the flowof heat via radiation, conduction, and convection into and out of thenon-working gas within the cylinder 901, thereby improving systemefficiency.

An external flow return 923 is a tube allowing non-working gas to flowfrom the upper end zone 915 to the lower end zone 916 to permit pressureequalization, which may be necessary to improve system efficiency. Analternative means for achieving this pressure equalizing gas flow, notshown in FIG. 9, is to provide an internal flow return in the form of apassageway inside the centering shaft 921, which then takes the form ofa hollow tube. The volume of the upper end zone 915 and lower end zone916 relative to the size of the working region (that is, the regionbetween piston position levels 0 and 3) may need to be adequately largein order to maintain system efficiency, by eliminating the requirementfor excessive forces to compress the gas in the end zones 915 and 916.To this end, the external flow return 923 may include one or moreexpansion chambers (not shown in FIG. 9) along its length.

In order to improve the rate of heat transfer through the walls of theheating zone 913 into the working gas 902, paddles 924 may be attachedto the pistons 903 and 904. These paddles 924 stir the working gas 902as the pistons 903 and 904 move relative to each other, thereby causingturbulence and motion of the working gas 902, and helping improve systemefficiency. The paddles 924 also improve the rate of heat transfer fromthe working gas 902 through the walls of the cooling zone 914. Thepaddles 924 may have a variety of shapes, consistent with not makingcontact with each other or with the other piston.

FIGS. 10-11B illustrate methods for reducing vibrations in a powerconversion system according to an embodiment of the invention. In FIG.10, two of the cylinder assemblies of the type shown in FIG. 5A (or anyother cylinder assemblies according to the invention) are arranged sothat their central axes are coincident and opposing. The motion of thepistons for the system of FIG. 10 is controlled by their powerconversion electronics such that the corresponding pistons move insynchronism in exactly equal and opposite movements. Thus, the twopiston assemblies 1003/1005 move toward or away from each other atexactly the same speed, and likewise the two piston assemblies 1004 movetoward (or away from) each other in synchronism. The upper end zone 1015is common to both sides of the engine, while there is a separate lowerend zone 1016 at each end. Such an arrangement may be referred as anengine with “horizontally opposed” cylinders; or more generally,“axially opposed” cylinders, since the common axis need not necessarilybe horizontal. A horizontal placement may have advantages forarrangement of the flow of combustion gases past the heating zones.

In FIGS. 11A and 11B, four of the cylinder assemblies of the type shownin FIG. 5A (or any other cylinder assemblies according to the invention)are placed side-by-side so that their central axes are parallel andarranged in a diamond pattern as viewed end-on (shown in FIG. 11B). Thecontrolling power electronics ensures that the pistons in cylinders Aand C move together in the same direction and in exact synchronism. Thepistons in cylinders B and D also move together in the same directionand in exact synchronism, but in exactly the opposite direction to thosein A and C, as indicated by the cross and dot vector notation of FIG.11B. In order to keep the heating zones in all four cylinders closetogether, the two pairs of cylinders may need to be displaced axiallyrelative to each other rather than having their ends coplanar.

The methods described above can be extended to the implementation of aninternal combustion generator, in accordance with an embodiment of theinvention. In a similar fashion to that described for FIGS. 3A-4, theelectrical arrangement of FIG. 4 may be used to perform cyclical energystorage for a mechanical piston arrangement of an internal combustioncycle. FIG. 13 is a cross-sectional view of one possible such mechanicalarrangement, which can be seen to incorporate features already explainedwith reference to FIGS. 3A, 5A, and 9. In FIG. 13, two concentric pistonassemblies 1303 and 1304 surround a centering shaft 1321 in a cylinder1301, as in FIG. 9. An arrangement corresponding to FIG. 3A could alsobe implemented, wherein the piston assemblies are physically separate,with or without a centering shaft. In FIG. 13, a fuel/air mixture is fedinto the working gas region 1302 via an inlet valve and port 1332, andan outlet valve and port 1333 allows for exhaust gas to be ejected. Aspark plug 1331 is located at the upper end of the working gas region1302. The walls of the working gas region 1302 are thermally insulated,and are made strong enough to withstand the forces associated withignition of the fuel/air mixture. Other features may be similar to thosedescribed above, including concentric shafts 1307 and 1308, permanentmagnet plates 1305 and 1306, drive windings 1309 and 1310, magneticfield return paths 1311 and 1312, and end zones 1315 and 1316. Exhaustport 1334 provides a means of escape for gas in region 1316, so thatexcessive compression forces are not required to compress the gas inregion 1316.

FIG. 14 shows a timing diagram that the internal combustion generator ofFIG. 13 may follow while performing an Otto cycle shown in the P-Vdiagram of FIG. 15, in accordance with an embodiment of the invention.Curve 1444 is the piston position profile for piston 1303, and curve1445 is the piston position profile for piston 1304, for a repeatingcycle A-B-C-D-A. The piston positions are indicated by position levels 0and 1 on the y-axis of FIG. 14, which correspond to cylinder positionsindicated in FIGS. 13.

Between times A and B of FIGS. 14 and 15, the inlet valve 1332 of FIG.13 is open, allowing a fuel/air mixture to be drawn into the working gasregion 1302 as piston 1303 is moved from position Level 0 to Level 1(curve 1444). During this segment, piston 1304 is held at position Level0 (curve 1445). The motion of piston 1303 for this segment is depictedas having a straight-line shape (curve 1444), although in practice themotion may be nonlinear.

Between points B and C of FIGS. 14 and 15, the working gas 1302 iscompressed as piston 1304 is moved from position Level 0 to Level 1(curve 1445), while piston 1303 remains at Level 1 (curve 1444). Atpoint C, spark plug 1331 initiates combustion of the working gas 1302,at which time the pressure of the working gas 1302 jumps immediately tothe higher level shown at C′ in the P-V diagram of FIG. 15.

Between points C and D of FIGS. 14 and 15, the working gas 1302 expands,exerting mechanical force on the pistons, and forcing piston 1304downward (curve 1445) while piston 1303 remains at position Level 1(curve 1444). Again, the motion of piston 1304 for this segment isdepicted as having a straight-line shape, although in practice themotion may be nonlinear. At point D, exhaust valve 1333 is opened, atwhich time the pressure of the working gas 1302 falls immediately to thelower level shown at D′ in the P-V diagram.

Between points D and A of FIGS. 14 and 15, the exhaust valve 1333remains open, and the burnt working gas 1302 is ejected as piston 1303is moved from position Level 1 to Level 0 (curve 1444), while piston1304 remains at Level 0 (curve 1445).

It can therefore be seen that embodiments according to the inventionprovide a variety of different possible ways of using electrical storageof the cyclical energy required by a thermal cycle, including externaland internal combustion generators, and electrically-driven heat pumps.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. A method for generating electrical energy using a thermal cycle of aworking gas, the method comprising: using the motion of a piston in acylinder, containing the working gas performing the thermal cycle, toelectromagnetically induce current in an electrical circuit coupled tothe cylinder; using the electrical circuit to store the electricalenergy, produced by the current induced in the electrical circuit, in anelectrical storage device; and using the electrical energy stored in theelectrical storage device to electromagnetically provide a motive forceto the piston; wherein cyclically using the electrical circuit to storethe electrical energy and using the stored energy to provide a motiveforce to the piston effect a net positive average power transfer intothe electrical storage device over the course of the thermal cycle.
 2. Amethod according to claim 1, wherein the electrical circuit comprises anelectronic power converter, the method further comprising: using theelectronic power converter to perform closed-loop electronic control ofthe motion of the piston.
 3. A method according to claim 2, wherein theelectronic power converter performs the closed-loop control based onelectrical signals related to the state of the working gas.
 4. A methodaccording to claim 3, further comprising: using at least one of atemperature sensor, a pressure sensor, and a position sensor to deliverthe electrical signals related to the state of the working gas to theelectronic power converter.
 5. A method according to claim 1, whereinthe thermal cycle receives heat from external combustion.
 6. A methodaccording to claim 5, wherein the thermal cycle approximates a Sterlingcycle.
 7. A method according to claim 5, wherein the thermal cycleapproximates a Carnot cycle.
 8. A method according to claim 5, furthercomprising: using compression and expansion of the working gas between afirst piston and a second piston to perform the thermal cycle.
 9. Amethod according to claim 8, wherein the electrical circuit comprises aset of windings coupled to the cylinder, the method further comprising:using the motions of a first permanent magnet attached to the firstpiston and a second permanent magnet attached to the second piston toelectromagnetically induce current in the set of windings.
 10. A methodaccording to claim 9, further comprising: using the motions of the firstpiston and the second piston to move the working gas along the cylinderto effect successive heat transfer with a heating zone and a coolingzone of the cylinder.
 11. A method according to claim 9 wherein at leastpart of the shaft of the first piston moves concentrically within ashaft of the second piston.
 12. A method according to claim 8, furthercomprising: using an electronic power converter to control timing of thethermal cycle by controlling the motions of the first piston and thesecond piston.
 13. A method according to claim 12, further comprising:using the electronic power converter to control the motions of the firstpiston and the second piston such that the working gas moves between aheating zone, a cooling zone, and a neutral zone of the cylinder.
 14. Amethod according to claim 8, further comprising: using a thermal shadeattached to the first piston or the second piston to insulatenon-working gas within the cylinder.
 15. A method according to claim 8,further comprising: using a paddle attached to the first piston or thesecond piston to create turbulence in the working gas.
 16. A methodaccording to claim 8, further comprising: using an external flow returnto flow non-working gas between a first end zone and a second end zoneof the cylinder.
 17. A method according to claim 8, further comprising:mounting the first piston and the second piston around a commoncentering shaft.
 18. A method according to claim 1, further comprising:cycling the working gas through an internal combustion cycle.
 19. Amethod according to claim 18, wherein the thermal cycle approximates anOtto cycle.
 20. A method according to claim 1, further comprising: usinga first cylinder and a second cylinder each operating according to themethod of claim 1 to generate electrical energy, the first cylinder andthe second cylinder being operated in axial opposition to each other.21. A method according to claim 1, further comprising using a firstcylinder, a second cylinder, a third cylinder, and a fourth cylinder,each operating according to the method of claim 1 to generate electricalenergy, the first cylinder, second cylinder, third cylinder, and fourthcylinder being operated in a bundle with parallel axes of the cylinders,two of the cylinders being operated antiparallel to the other twocylinders of the first cylinder, second cylinder, third cylinder, andfourth cylinder.
 22. A method for powering a heat pump using electricalenergy, the heat pump performing a thermal cycle, the method comprising:using electrical energy stored in an electrical storage device toelectromagnetically provide a motive force to a piston in a cylindercontaining the working gas performing the thermal cycle; using themotion of the piston to electromagnetically induce current in anelectrical circuit coupled to the cylinder; and using the electricalcircuit to store the electrical energy, produced by the current inducedin the electrical circuit, in the electrical storage device; whereincyclically using the stored energy to provide the motive force to thepiston and using the electrical circuit to store the electrical energyeffect a net positive average power transfer out of the electricalstorage device over the course of the thermal cycle.
 23. A methodaccording to claim 22, wherein the electrical circuit comprises anelectronic power converter, the method further comprising: using theelectronic power converter to perform closed-loop electronic control ofthe motion of the piston.
 24. A method according to claim 23, whereinthe electronic power converter performs the closed-loop control based onelectrical signals related to the state of the working gas.
 25. A methodaccording to claim 24, further comprising: using at least one of atemperature sensor, a pressure sensor, and a position sensor to deliverthe electrical signals related to the state of the working gas to theelectronic power converter.
 26. A method according to claim 22, whereinthe thermal cycle receives heat from external combustion.
 27. A methodaccording to claim 26, wherein the thermal cycle approximates a Sterlingcycle.
 28. A method according to claim 26, wherein the thermal cycleapproximates a Carnot cycle.
 29. A method according to claim 26, furthercomprising: using compression and expansion of the working gas between afirst piston and a second piston to perform the thermal cycle.
 30. Amethod according to claim 29, wherein the electrical circuit comprises aset of windings coupled to the cylinder, the method further comprising:using the motions of a first permanent magnet attached to the firstpiston and a second permanent magnet attached to the second piston toelectromagnetically induce current in the set of windings.
 31. A methodaccording to claim 30, further comprising: using the motions of thefirst piston and the second piston to move the working gas along thecylinder to effect successive heat transfer with a heating zone and acooling zone of the cylinder.
 32. A method according to claim 30 whereinat least part of the shaft of the first piston moves concentricallywithin a shaft of the second piston.
 33. A method according to claim 29,further comprising: using an electronic power converter to controltiming of the thermal cycle by controlling the motions of the firstpiston and the second piston.
 34. A method according to claim 33,further comprising: using the electronic power converter to control themotions of the first piston and the second piston such that the workinggas moves between a heating zone, a cooling zone, and a neutral zone ofthe cylinder.
 35. A method according to claim 29, further comprising:using a thermal shade attached to the first piston or the second pistonto insulate non-working gas within the cylinder.
 36. A method accordingto claim 29, further comprising: using a paddle attached to the firstpiston or the second piston to create turbulence in the working gas. 37.A method according to claim 29, further comprising: using an externalflow return to flow non-working gas between a first end zone and asecond end zone of the cylinder.
 38. A method according to claim 29,further comprising: mounting the first piston and the second pistonaround a common centering shaft.
 39. A method according to claim 22,further comprising: cycling the working gas through an internalcombustion cycle.
 40. A method according to claim 39, wherein thethermal cycle approximates an Otto cycle.
 41. A method according toclaim 22, further comprising: using a first cylinder and a secondcylinder each operating according to the method of claim 22 to power aheat pump using electrical energy, the first cylinder and the secondcylinder being operated in axial opposition to each other.
 42. A methodaccording to claim 22, further comprising using a first cylinder, asecond cylinder, a third cylinder, and a fourth cylinder, each operatingaccording to the method of claim 22 to power a heat pump usingelectrical energy, the first cylinder, second cylinder, third cylinder,and fourth cylinder being operated in a bundle with parallel axes of thecylinders, two of the cylinders being operated antiparallel to the othertwo cylinders of the first cylinder, second cylinder, third cylinder,and fourth cylinder.